Currently, many technologies exist to measure the pressure of two contact surfaces. However, fewer options are available when interested in creating low cost arrays of pressure measurement elements to determine the pressure distribution. In these cases, the most common options are based on creating arrays of capacitive, resistive or piezo-electric components. A conventional capacitive measuring pad is constructed of transverse conductive strips separated by a compressible insulator to form a matrix of pressure sensitive capacitive nodes. The nodes are repetitively scanned in sequence by a microcomputer to measure their respective capacitances, from which measurements a pressure map is then derived. The resulting pressure map may be displayed on a color graphics monitor with different colors representing different pressures.
Each of these technologies operates similarly from the point of view that an electrical signal is injected into the array and a measurement is performed on the subsequent output signal from the array. As well, in each method, a specific node under consideration must be separated or isolated from the other nodes in the matrix. This is often referred to as multiplexing and can be performed on either the inputs or outputs of the array or both. The actual process of isolating the node from the other nodes may vary as there are alternative solutions to this problem.
Node capacitance is found by measuring the response of the node to a driving signal of a known voltage. This measurement is accomplished by connecting one of the node's transverse conductive strips (columns) to the driving source and the node's other conductive strips (rows) to a sense amplifier. In order to isolate the node of interest from the influence of surrounding nodes, all of the columns and rows except the two intersecting the selected node are connected to ground. An excite signal is then injected onto the desired ungrounded column and the received signal is measured from the desired ungrounded row. The measuring step is repeated for each node on the row. Once each node along a row is measured, the desired row is changed and the measurement process is repeated until all nodes have been measured. This yields a matrix of measurement data called a frame with the number of measurement elements equal to max[column] multiplied by max[row].
By previously determining the relationship between pressure and received signal amplitude for a sensor through a process called calibration, the pressure at each capacitive node or sensor cell can be estimated for each frame, thereby yielding a pressure image of any objects making contact with the sensor between them.
However, grounding of the unmeasured columns and rows does not perfectly isolate the node of interest from the effects of the other nodes unless a zero impedance arrangement such as a current-sensing amplifier is used to measure the output of the matrix, as is the case in U.S. Pat. No. 5,010,772 (the contents of which are incorporated herein by reference). The input impedance to ground of the sense amplifier is made negligibly small with respect to the other system impedance. In this way, only the column connected to the driving source has a voltage impressed on it, and the other columns of all other nodes in the system are maintained at ground potential, thus allowing an accurate measurement of the one capacitance. However, even with grounding of the sense amplifier, when the other nodes have changing pressure, and hence changing capacitance, there is still an error in the selected node measurement unless the input impedance of the signal sensing circuitry is identically zero. Compounding the non-zero input impedance error is the fact that the magnitude of the error is a dynamic variable based on what pressure is being applied to the other unused nodes.
Therefore, there is a need in the art for alternative methods of measuring individual node capacitance, in a pressure mapping system.